Projectsummary Theabilityofskeletalmuscletogrowandrespondtoinjuryanddiseaselargelydependsonapopulationof myogenicstemcells,calledsatellitecells.Indevelopingorinjuredmuscle,satellitecellsarehighly-activated andproliferative.Somedaughtercellsthenundergoacomplexprogramofdifferentiationthatisnecessaryto establishormaintainmusclehealthandfunction.Perturbationsofmechanismsthatregulatesatellitecell proliferationordifferentiationcanleadtolossofnormalmusclefunctionand,insomecases,reducelifespan. WehavediscoveredthattheproteinKlothoregulatessatellitecelldevelopmentinyoung,healthymuscle.As weexploredthemechanismsthroughwhichKlothoaffectssatellitecells,welearnedthatKlothoreduces expressionofanenzyme,Jmjd3,thatactivatesgenesbyremovingasilencingmarkfromlysine27onhistone 3(H3K27).Inaddition,theKlotho-inducedreductionsinJmjd3areassociatedwithincreasedH3K27 methylationandreducedexpressionofWnt-relatedgenesthatregulatemyogenesis.Together,the observationssuggestanovelmechanismthroughwhichmyogenesisisregulated,involvingsignalingthrougha Klotho/Jmjd3/Wnt-mediatedpathway.Furthermore,thepotentialbiologicalimportanceofthepathwayis emphasizedbyourfindingsthatthetargeteddeletionofJmjd3insatellitecellsproducedreductionsinWnt signaling,defectsinmyogenesisanddeathinneonatalmice. ThecentralhypothesisofourproposedinvestigationisthatsignalingthroughtheKlotho/Jmjd3/Wntpathway playsanimportantroleinmyogenesisbyregulatingsatellitecellnumbersanddifferentiation.Furthermore,we proposethatthispathwayisconservedthroughoutlife,beginningwiththefirstappearanceofsatellitecellsat thefetalmusclefibersurfaceandcontinuingintoadulthood,whensatellitecellsareactivatedbyinjuryor disease.WeproposetotestthathypothesisthroughthefollowingSpecificAims. Aim1.DeterminetheeffectsofmodulatingKlothoandJmjd3onpost-natalmusclegrowthanddifferentiation. Aim2.DeterminetheeffectsofmodulatingKlothoandJmjd3onmuscleregenerationfollowingacuteinjury. Aim3.DeterminethefunctionofsatellitecellJmjd3inlate,fetal,muscledevelopment. Theproposedexperimentswillestablishtheimportanceofanovelmechanismthatregulatessatellitecell developmentandresponsetomuscleinjury.Webelievethattheoutcomesofthisinvestigationwilldeepenour knowledgeofvitalmechanismsthatregulatemuscledevelopmentandadaptationandwillalsoprovidenew insightsintotheresponseofmuscletoinjuryanddisease.